Spectral imaging is a form of imaging in which a defined band of electromagnetic energy is used to generate an image of an object. The image is referred to as a spectral image. Multispectral imaging is a technique in which multiple bands of electromagnetic energy are used to generate multiple images of an object. That is, each band of electromagnetic energy generates a corresponding one of the multiple images. The multiple images are also referred to as spectral images. Some forms of multispectral imaging use very narrow passbands to form the resulting spectral images in which a corresponding small range of light wavelength can be discerned.
There are two fundamental types of conventional multispectral imaging. A “scanning” type of multispectral imaging scans an object in wavelength and/or in space to provide the plurality images. The conventional scanning type of multispectral imaging requires a substantial time to generate the plurality of images, and therefore, is not suitable for all applications, for example, applications in which the object is moving. A “simultaneous acquisition” type of multispectral imaging is able to generate the plurality of images simultaneously or nearly simultaneously. The conventional simultaneous acquisition type of multispectral imaging can generate the plurality of spectral images quickly, however, is only able to generate a relatively small number of spectral images.
With respect to the scanning type of multispectral imaging, there exist a variety of sub-types, for example, spectral scanning, spatial scanning, spectral-spatial scanning, interferrometric scanning, and light source spectrum scanning. Each of these sub-types requires a substantial amount of time to generate a plurality of spectral images.
Spectral scanning uses a plurality of narrowband optical filters positioned one at a time between an object and a light detector. By selectively changing the optical filter and associated transmission wavelength, spectral scanning generates one spectral image at a time.
Spatial scanning uses a line scan, for which a line of the line scan includes information at a plurality of wavelengths. The line scan is spatially scanned across the object to generate a plurality of spectral images.
Spectro-spatial scanning uses a linear variable interference filter (LVIF). An LVIF is an optical band bass filter having a central wavelength that changes along one spatial dimension. Like the above spectral scanning, by selectively scanning the LVIF and associated transmission wavelength, spectro-spatial scanning can generate one spectral image at a time.
Interferometric scanning uses a Fourier transform spectrometer in a Twyman-Green arrangement, in which a mirror is moved to scan an object. After completion of the scanning and acquisition, resulting data is analyzed (inverse Fourier transformed) to generate a plurality of spectral images.
Light source spectrum scanning uses a light source having a selectively variable output wavelength to illuminate an object. Like the above spectral scanning, by selectively changing (i.e., scanning) the light source transmission and/or emission wavelength, light source spectrum scanning can generate one spectral image at a time.
With respect to the simultaneous acquisition type of multispectral imaging, there also exist a variety of sub-types, for example, a multiple parallel camera type, an image cloning type, a dispersed optics type, a multiple microfilter type, and an unconventional CCD type. Each of these sub-types is able to generate only a relatively small number of spectral images.
Multiple parallel camera multispectral imaging uses a plurality of cameras to simultaneously generate spectral images of an object. Each one of the cameras captures an image at a different wavelength. However, because the cameras have a relatively large physical extent, they are unable to capture exactly the same perspective of the object, and therefore, lack image registration. Also, each one of the cameras, having a different lens, must be separately focused. The number of spectral images is limited by the number of cameras.
Image cloning multispectral imaging uses an optical assembly (e.g., a plurality of prisms) to split an image from an object into a plurality of image clones. Each one of the image clones can be passed through a separate optical filter prior to detection. This technique is used in many conventional CCD cameras. Existing conventional arrangements include systems that direct all of the image clones onto separate portions of a single CCD and systems that direct the image clones onto separate CCDs. The image can be split into the image clones by sets of prisms, mirrors, beam splitters, or focusing mirrors. Each image has an intensity less than the original image intensity (i.e., it is degraded), in inverse proportion to the number of image clones generated. The number of spectral images that can be generated is limited by the number image clones.
Dispersed optics multispectral imaging passes light received from the object through a grating in order to form several spatially distorted and overlapping spectral images on a light detector. This type has poor imaging resolution.
Multiple microfilter multispectral imaging uses sets of micro-filters arranged in a so-called color filter proximate to a CCD array in order to measure colors. The color filter usually has a combination of three or four different broadband micofilters arranged in a grid pattern. Each pixel of the CCD array is covered by one of the relatively broad bandpass micofilters. Each one of the broad bandpass micofilters passes light having a color corresponding to the respective microfilter. The combination of three or four different broadband microfilters spread out in a grid pattern proximate to the CCD array can simultaneously capture a plurality of colors at every part of the image. However, the number of spectral images is limited by the number of different microfilters in the color filter. Also, because the spectral bands of the microfilters in the color filter tend to be relatively broad, the wavelength selectivity of a resulting spectral image is limited.
Conventional color CCDs used in multiple microfilter multispectral imaging are used in conjunction with either a cyan-magenta-yellow-green (CMYG) or a red-green-blue (RGB) set of microfilters arranged in the above-described grid pattern to form a color optical filter. For example, a CMYG CCD has groups of four adjacent pixels covered with different microfilters and all four pixels measure essentially the same spatial point on an object but in different spectral bands. As used herein, the CCD is referred to separately from the associated color optical filter, though they tend to be joined into a common assembly in a conventional color CCD.
Unconventional CCD multispectral imaging uses a type of CCD having CCD pixels at different depths in a substrate. The different depths form bandpass filters (i.e., microfilters) with respective spectral bands (i.e., central wavelengths) related to their depths. This arrangement is similar to the above-described multiple microfilter multispectral imaging, but the microfilters are formed by the depths. The number of spectral images is limited by the number of different depths. Also, because the spectral bands corresponding to the depths tend to be relatively broad, the wavelength selectivity of a resulting spectral image is limited.